Shyncronous Machine

Synchronous motors convert electrical power to mechanical power, synchronous generators convert mechanical power to electrical power and synchronous condenser supply only reactive power to stabilize system.

Synchronous Machines, in General
This section covers performance common to synchronous motors, generators, and condensers. Saturation curves are either calculated or obtained from test and are the basic indicators of machine design suitability. From these the full load field, or excitation, amperes for either motors or generators are determined as shown, on the rated voltage line, as ‘‘Rated Load.’’ For synchronous condensers, the field current is at the crossing of the zero P.F. saturation line at 1.0 V.


As an approximate magnetic figure of merit, the no-load saturation curve should not exceed its extrapolated straight line by more than 25%, unless of a special design. From these criteria, and the knowledge of the stator current and cooling system effectiveness, the manufacturer can project
the motor component heating, and thus insulation life, and the efficiency of the machine at different loads.





Vee curves show overall loading performance of a synchronous machine for different loads and power factors, but more importantly show how heating and stability limit loads. For increased hydrogen pressures in a generator frame, the load capability increases markedly.






Synchronous Generator Capability
The synchronous generator normally has easy starting duty as it is brought up to speed by a prime mover. Then the rotor excitation winding is powered with DC current, adjusted to rated voltage, and transferred to voltage regulator control.


It is then synchronized to the power system, closing the interconnecting circuit breaker as the prime mover speed is advancing, at a snail’s pace, leading the electric system. Once on line, its speed is synchronized with the power
system and KW is raised by increasing the prime mover KW input. The voltage regulator adjusts excitation current to hold voltage. Increasing the voltage regulator set point increases KVAR input to the system, reducing the power factor toward lagging and vice versa. Steady operating limits are provided by its Reactive Capability Curve .

This curve shows the possible kVA reactive loading, lagging, or leading, for given KW loading. Limitations consist of field heating, armature heating, stator core end heating, and operating stability over different regions of the reactive capability curve.

Synchronous Motor and Condenser Starting
The duty on self-starting synchronous motors and condensors is severe, as there are large induction currents in the starting cage winding once the stator winding is energized These persist as the motor comes up to speed, similar to but not identical to starting an induction motor.

Similarities exist to the extent that extremely high torque impacts the rotor initially and decays rapidly to an average value, increasing with time. Different from the induction motor is the presence of a large oscillating torque. The oscillating torque decreases in frequency as the rotor speed increases.

This oscillating frequency is caused by the saliency effect of the protruding poles on the rotor. Meanwhile, the stator current remains constant until 80% speed is reached. The oscillating torque at decaying frequency may excite train torsional natural frequencies during acceleration, a serious train design consideration. An anomaly occurs at half speed as a dip in torque and current due to the coincidence of line frequency torque with oscillating torque frequency. Once the rotor is close to rated speed, excitation is applied to the field coils and the rotor pulls into synchronism with the rotating electromagnetic poles. At this point, stable steady-state operation begins.

Increasingly, variable frequency power is supplied to synchronous machinery primarily to deliver the optimum motor speed to meet load requirements, improving the process efficiency. It can also be used for soft-starting the synchronous motor or condenser. Special design and control are employed to avert problems imposed, such as excitation of train torsional natural frequencies and extra heating from
harmonics of the supply power.

From Leonard L Grigsby GENERATION,TRANSMISSION AND DISTRIBUTION CRC Press